We are studying the human opsin promoter in transgenic mice. To map the cis regulatory elements responsible for opsin gene expression in rod photoreceptor cells, fusion genes containing either 1,000 or 600 bp of sequence flanking the 5' region of the opsin gene has been placed upstream of the bacterial chloramphenicol acetyl transferase (CAT) gene. A hybrid gene containing 200 bp of 5' flanking and 40 bp of exon 1 of the human opsin gene (Nathans and Hogness, Proc Natl Acad Sci USA 1984;81:4851) fused to the CAT gene was microinjected into fertilized mouse eggs and several live births are being analyzed to determine whether the injected gene has become integrated into the mouse genome.